1. Field of the Invention
The present invention relates to a numerical control apparatus having a thread cutting function and a spindle rotational speed override function.
2. Description of the Prior Art
In performing thread cutting with a lathe, cutting feeds are applied in several batches while a workpiece to be cut is being rotated by a spindle. When spindle rotational speed is changed by means of overriding during the thread cutting, a speed of a feed axis for moving a cutting tool changes, and a follow-up error in the feed axis changes. As a result, a relationship between a threaded portion and the cutting tool unavoidably changes, causing a problem that thread cutting is difficult to perform exactly.
To solve such a problem, there has been developed a method of converting a variation of a follow-up error in a feed axis, which is caused by a difference between a spindle rotation command value commanded according to a processing program and a spindle rotational speed during processing changed by means of overriding, into a spindle angle, and of shifting the spindle angle at a thread cutting start by an amount of the converted spindle angle, so as to control the thread cutting start, as disclosed in Japanese Patent Laid-Open No. 2000-176789.
The above-mentioned method determines the variation of the follow-up error in the feed axis caused by a change in the spindle rotational speed applied with overriding into the spindle angle by converting the variation into the spindle angle. A thread cutting is started from a position shifted from the spindle angle. Therefore, the precision of the thread cutting start position depends upon the resolution of a detector for detecting the spindle rotational angle, thus impairing the precision of the thread cutting.